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As noted by Jon, you (most commonly) use a DHCP server's IP but you can use other IPs. It depends on what broadcasts you want to send to an off local network host. IP helper, by default, only repackages some broadcasts, but you can extend what IP helper uses by using the global command ip forward-protocol.
For example, using a combination of ip foward-protocol and ip helper, I can have out-of-the-box WLAPs contact a controller of my choosing.
You might also use ip helper for diskless workstation bootp requests.
